New nucleoside and nucleotide analogues will be prepared and studies as antiviral and antitumor agents. These compounds will include: (i) enantiomeric antiviral methylenecyclopropane nucleoside analogues, (ii) lipophilic phosphate prodrugs, phosphonates and triphosphates of methylenecyclopropane analogues, (iii) methylenecycloalkane nucleoside analogues with cyclobutane, cyclopentane,and cyclohexane ring systems and (iv) gem-difluoromethylenecyclopropane analogues. Antiviral effects will be studied in a number of viral assays including human and murine cytomegalovirus (HCMV and MCMV), herpes simplex virus type 1 and 2 (HSV-1 and HSV-2) and human immunodeficiency (HIV). In vivo activity and mechanism of action of active compounds will also be investigated. Selected analogues will also be studies as potential antitumor agents.